The present invention relates to a color image forming apparatus for preventing a deterioration in picture quality and the wasteful consumption of a toner when carrying out printing in a monochrome mode in a structure in which the power sources of a charging device and a developing device are shared.
A color image forming apparatus of a tandem type is provided with four image units for forming monochromatic toner images of yellow (Y), magenta (M), cyan (C) and black (Bk). A photosensitive drum or the like served as a charging device, a developing device and a latent image carrier is provided in each of the image units. The charging devices and the developing devices for the respective colors are connected to power sources provided individually. The color image forming apparatus is used in a color mode and a monochrome mode.
In the color image forming apparatus having the above structure, a charging potential to be desired for charging the latent image carrier is high in some cases. In these cases, a large quantity of charge is to be supplied. For this reason, a memory is apt to be generated on the latent image carrier due to an insufficient charging capability. In the case in which the memory is generated on the latent image carrier, moreover, a variation in the potential is increased if the charging potential is high. Consequently, a deterioration in picture quality becomes more remarkable.
In the case in which an operation is to be carried out in the color mode, particularly, a gradation property is important for the picture quality. Therefore, there is such a demand that the charging potential is to be lowered as much as possible. If the charging potential is too low, however, an inverse contrast to the developed portion of a latent image is reduced so that so-called “scumming” on a white background is generated in a recording medium such as a recording paper. Therefore, the charging potential is set to a voltage so that the “scumming” is not generated.
In the case in which the charging potential is set to be such a voltage that the “scumming” is not generated, thus, the contamination of the recording medium cannot be visually confirmed. In the case in which the image forming apparatus is operated in the color mode, however, it is confirmed that a small amount of toner is consumed and is stuck onto the recording medium if a concentration measurement and a microscopic observation are carried out. For this reason, there is still a problem in that the picture quality is deteriorated and the toner is wastefully consumed.
In the color image forming apparatus in which the color mode and the monochrome mode are performed together, printing is often carried out in the monochrome mode in which only characters are actually printed. There is a problem in that the picture quality is deteriorated and the toner is wastefully consumed as described above when the printing in the monochrome mode is carried out while a color image forming section is operated. For this reason, in the case in which the printing is carried out in the monochrome mode, a power source connected to the color image forming section is turned OFF or the operation of a developing unit is stopped.
However, there is a problem in that a control circuit is complicated when a control for turning OFF the power source of the color image forming section and a processing of stopping the operation of the developing unit are carried out. With such a structure that the charging devices and the developing devices for the respective colors are connected to the power sources provided individually, moreover, there is a problem in that the cost of power equipment is increased.
In order to take a countermeasure against these problems, there has been such a trial as to share the power sources of the charging device and the developing device which are provided for each color, thereby reducing the expense of the power equipment (for example, see JP-A-2002-162801). In this case, the same voltage is applied from the common power source to the charging devices for the respective colors. Moreover, the same voltage is applied from a common power source to the developing devices for the respective colors. Therefore, there is an advantage that the cost of the power equipment can be reduced.
In the structure disclosed in JP-A-2002-162801, printing in a monochrome mode is carried out while a color image forming section is operated since a common power source for each color is used. For this reason, there is a problem in that the picture quality is deteriorated and the toner is unnecessarily consumed.